Method and apparatus for concentrating solutions



July 9, 1957 5 300 2,798,543

METHOD AND APPARATUS FOR CONCENTRATING SOLUTIONS Filed Sept. 3, 1953 INVENTOR United States Patent METHOD AND APPARATUS FOR CONCENTRATIN G SOLUTIONS Robert S. Cook, Bulfalo, N. Y.

Application September 3, 1953, Serial No. 378,353

4 Claims. (Cl. 159-27) The invention relates to improvements in the concentration of industrial solutions through forced evaporation of the solvent and in particular to the evaporation of the solvent from solutions whose boiling point at constant pressure increases with concentration, and during the concentration of which corrosive products are formed and released as the process passes through a critical temperature range.

An example of the problem is in the commercial evaporation of water from caustic soda solutions, in which the concentration to seventy or seventy-five percent NaOH as presently practiced in nickel tubular evaporators presents no serious problems of corrosion. The average grade of 50% liquid caustic which is now marketed contains minor amounts of chlorate of sodium which is not atfected by the temperature of evaporation to 70-75%. However, if the evaporation is carried further, the boiling temperature rapidly increases and reaches a rang Where the chlorate is decomposed and severely attacks the heating surface. To overcome this detriment, the accepted practice at present is to submit the solutions to a purification process prior to the evaporation above the 70-75% concentration. With the chlorate thus removed, dehydration can be carried through to concentrations of 95% or more without serious attack on the heating surface.

One of the objects of the present invention is to eliminate the necessity of prior purification processes to avoid the severe attack on the heating and other surfaces of the evaporator and consequent excessive contamination of the product.

Another object of the present invention is to remove the limitation of operating present evaporators at approximately 20" of vacuum or less to prevent plugging of the tubes due to the freezing point of the solution rising above the boiling point in the vicinity of 85% concentration. This invention removes this limitation with consequent reduction in the heat requirement for the process.

Other improvements over existing processes and apparatus for the service may be apparent from the description which refers to the accompanying drawing, in which:

The figure represents a conventional assembly of a tubular heat exchanger 2, liquor and/ or vapor body 3, and vapor condenser 4, the assembly being commonly called an evaporator. The preheater 5 is a commonly known piece of equipment frequently identified with evaporators of this type, but is not always an essential part of the sysetm. The feed pump 6, storage tank 7, and other numerous auxiliaries are needed for the present description of the process. Their function will become apparent.

The liquor and vapor body 3 is made up of one or more cylindrical sections with their longitudinal dimension vertical. The body 3 can be divided into two or more sections by longitudinal partitions. In this drawing and process, the body 3 is divided into two compartments from an elevation near the bottom to an elevation extending well into the vapor space 8. The bottom edge of the partition is spaced apart from the bottom of the body 3 so that the two compartments are open to each other below the par- A 2,798,543 Patented July 9, 1957 tition 9. Somewhere in the lower part of the partition 9 is an opening which connects to the suction inlet of a jet pump 10. The pump 10 is arranged so that it discharges into the compartment opposite that to which its suction is connected.

The weak liquor enters the system through a pump 6, passing through a preheater 5 and then as driving fluid is introduced into the jet pump 10 which aspirates a quantity of liquor at final concentration and temperature from a second evaporator section 11, discharging the mixture into a first evaporator section 12 containing a liquor of intermediate composition at its boiling point which is above the decomposition temperature of the chlorate.

The liquor in the first evaporator section 12 then circulates downward through the lower part of the first evaporator section 12 which communicates near its lower extremity with the similar evaporator section 11 containing liquor of final concentration and temperature, the two chambers having a common vapor space 3 connected to a single condenser 4.

The lowermost part of the two evaporator sections merge into a single conduit 13 which conducts liquor from both chambers into the bottom of a conventional tubular heat exchanger 2 which heats the mixed. liquors of final and inetrmediate composition, causing the desired vaporization of the residual amount of solvent from a composition of liquor in which the chlorate has been substantially decomposed. Vapor and liquor at the final concentration issue from the top of the heat exchanger, discharging into the second evaporator chamber 11 wherein the vapor separates from the liquor at its final concentration. The net accumulation of finally concentrated liquor overflows from the second evaporator chamber H to a suitable storage tank 7.

There are numerous arrangements which the jet pump 10 may take. One such which might be used to advantage is to have it extend through the partition with its suction in the second evaporator section 11 and with the weak liquor feed line passing through the liquor of final concentration and temperature to the nozzle of the jet pump, as this arrangement would give more preheating of the weak liquor before entering the pump in cases Where this might be desirable. Another such arrangement would be to have the jet pump outside the evaporator sections for reasons of inspection or maintenance.

The objectives of the invention are attained through the use of the double section evaporator body and the jet pump. Neither alone would be sufiicient to give the desired results. Although other types of pumps could be uesd in theory to accomplish the same result as with the jet pump, the latter is uniquely suited to give the desired results.

To reduce the corrosive attack on the equipment, it is necessary to carry the process through the critical temperature range without transferring heat to it through a separating surface of material such as the nickel tubes in the heat exchangers customarily used in this service. The jet pump fulfills this requirement by allowing a large quantity of comparatively inert liquor of final concentration to envelope and transfer heat to the weak liquor as it is concentrated through the critical temperature range. Since the reaction of chlorate decomposition may not be completed within the pump, the second body of solution is provided to insure substantial completion before the nascent products can reach a major surface of the equipment on which to act.

The jet pump also overcomes the tendency toward solidification under high vacuums at the critical concen tration. It provides the means for enveloping the critical concentration in a preponderance of higher temperature liquor under conditions of velocity so as to make the phase of critical composition extremely transitory and the mixture is well on its way out of the pump before it can propagate any solid phase into a blocked passage as happensin the tubes of an exchanger. Therefore,v the pressure in the vapor space may be held lower thanwould be possible in evaporators now used for the process.

The preheater 5 is included as a means for controlling the concentration of the solution in the first evaporator section 12. By varying the amount of heat supplied to the weak liquor in this exchanger, the resultant concentration in the first evaporator section will be changed. This is readily adaptable to automatic control by temperature, as

is the heat supply to the heat exchanger of the evaporator,

which is regulated to hold the final concentration of liquor in the second evaporator section.

In the application of the apparatus to the process described, the first section 12 will be larger than the second section, due to the much larger amount of evaporation taking place in the first section. Also, the partition should not pass and does not pass through the vapor inlet from the vaporizing heat exchanger, as the liquor level will be higher in the first section than in the second.

I claim:

1. Apparatus for the concentration of solutions comprising -a body for liquor and vapor divided into two sections, each comprising a liquor chamber and a vapor chamber said vapor chambers communicating with each other, said liquor chambers being isolated from each other by a partition dividing the body into said sections, a conduit leading from a first of the liquor chambers to a surface type heat exchanger with exit end of said heat exchanger communicating with the vapor chamber above the second of the liquor chambers, a liquid jet pump with inlets for receiving aspirated liquor from the second liquor chamber and an aspirating weak feed liquor, with discharge end of said pump communicating with the first liquor chamber.

2. The method of concentrating solutions which comprises injecting a weak solution as the driving fluid into a jet pump, aspirating solution of higher concentration and temperature from a body of concentrated solution into admixture with said weak solution, said concentrated solution being in communication with a heated supply of said last mentioned concentrated solution and a vapor removal chamber, discharging the mixture into another body of solution of intermediate concentration in communication with its vapor removal chamber, removing vapor from said mixture, leading a portion of the resulting mixture conjointly with a portion of the body of concentrated solution to a heat supply to produce a final mixture of vapor and solution and thus said heated supply, conducting said final mixture of vapor and solution from said heated supply into the vapor removal chamber above the body of concentrated solution, effecting separation of vapor from solution therein, removing net accumulation of concentrated solution from the body of concentrated solution and supplying heat to said weak solution in controlled variable amount.

3. Apparatus for the concentration of solutions comprising a body for liquor and vapor divided into two sections, each comprising a liquor chamber and a vapor chamber, a common liquor chamber between the sections at their lower extremity, a conduit leading from said lower extremity to a heat exchanger with exit end reconnecting to the vapor chamber of a second of said sections, said vapor chamber communicating freely with the vapor chamber of a first of said sections, the liquor chamber of said second section communicating with the suction of a liquid jet pump, said pump having an inlet for motive fluid and a discharge communicating with the first of said sections.

4. The method of concentrating solutions which comprises mixing of a substantially continuous flow of weak solution wth solution of final concentration, heating said weak solution so as to cause vaporization of solvent from the resultant mixture prior to further exposure to a heat supply, conducting the resulting solution of intermediate concentration to further admixture with solution of final concentration, and supplying sutlicient heat to the lastly formed mixture as to cause further vaporization of solvent and thereby giving solution of final concentration.

References Cited in the file of this patent UNITED STATES PATENTS 428,281 Miller May 20, 1890 521,946 Cooper June 26, 1894 690,981 Morgan Jan. 14, 1902 1,028,738 Kestner a- June 4, 1912 1,057,566 Marsh et a1. Apr. 1, 1913 1,331,373 Prache Feb. 17, 1920 1,469,475 Wirth-Frey Oct. 2, 1923 1,478,990 Weil Dec. 25, 1923 1,945,281 Leithauser Jan. 30, 1934 2,022,037 Hanchett Nov. 26, 1935 FOREIGN PATENTS 362 Great Britain Jan. 8, 1890 75,203 Germany Jan. 10, 1919 562,476 France Nov. 12, 1923 427,780 Germany July 29, 1926 531,224 Germany Aug. 6, 1931 

